Liquid crystal display device and method for fabricating the same

ABSTRACT

A liquid crystal display device and a method of fabricating the same are disclosed. The device, as disclosed, reduces the number of masks required for fabrication. Thus, cost is reduced and yield is increased. The device includes a plurality of gate lines, a plurality of data lines crossing the gate lines such that active regions are defined near the crossover points, thin film transistors are formed near the active regions, and a plurality of pixel electrodes are formed within regions defined by the adjacent gate and data lines. Also, pixel electrodes overlap gate lines and the two electrodes function as a storage capacitor. A fabrication method includes forming a gate line; forming a data line region, protection layers, and an active area where drain and source electrodes are spaced apart at a predetermined distance; forming a data line, a gate line protection layer, and a gate insulating layer; and forming pixel electrodes by depositing a transparent conductive material, such that each pixel electrode also overlaps a portion of a gate line.

This application claims the benefit of Korean Patent Application No.1999-38017, filed on Sep. 8, 1999, which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display (LCD) device,and more particularly, to a device and method for fabricating the LCDdevice having a thin film transistor (TFT).

2. Discussion of the Prior Art

Generally, an LCD device includes top and bottom glass substrates and aliquid crystal injected therebetween. On the bottom glass substrate, aplurality of gate lines extending in one direction and a plurality ofdata lines extending in a perpendicular direction are formed. In thismatrix arrangement, a plurality of TFTs are disposed near the crossoverpoints of the data and gate lines.

On the top glass substrate, red (R), green (G) and blue (B) color filterlayers and a common electrode are disposed. Generally, a light shieldinglayer (black matrix) is formed on the top glass substrate and a pair ofpolarizers are disposed on the outer surfaces of the top and bottomglass substrates to selectively transmit light.

A conventional LCD device will be described in detail below withreference to FIG. 1, which is a plan view of a conventional LCD device.

As illustrated in FIG. 1, the conventional LCD device includes aplurality of gate lines 22 formed on a transparent substrate, aplurality of data lines 24 perpendicularly crossing the gate lines 22, aplurality of TFTs “S” formed near the crossover points of the gate anddata lines 22 and 24, and a plurality of pixel electrodes 14 connectedto the TFTs “S”. The gate lines 22 are separated by intervals from eachother and extend in one direction, whereas the data lines 24 areseparated by intervals from each other and extend in a perpendiculardirection to the gate lines 22. Each end portion of gate and data lines22 and 24 has gate and data pads 21 and 23, respectively. A storagecapacitor “Cst” is arranged on a predetermined portion of the gate line22. Two adjacent gate lines 22 and two adjacent data lines 24 define theboundaries of a pixel region. In each pixel region, a TFT “S” and apixel electrode 14 are disposed.

Each TFT includes a gate electrode 26, a source electrode 28 and a drainelectrode 30. A gate insulating layer is formed between the gate andsource electrodes 28 and 30 and between the gate and drain electrodes 26and 30. The gate electrode 26 extends from the gate line 22 and thesource electrode 28 extends from the data line 24. The drain electrode30 connects the pixel electrode 14 through a contact hole 31.

The TFT transmits a signal of the data line 24 to the pixel electrode 14in response to a signal of the gate line 22.

In the conventional LCD device having the above-described TFTs, if asignal voltage is applied to the gate electrode 26, the TFT is turned onso as to transmit a data voltage representing picture data to the pixelelectrode 14 and the liquid crystal.

FIGS. 2A to 2E show fabrication process steps of an active matrix liquidcrystal display device according to the conventional art.

First, a first metal layer is deposited on a substrate 1 by a sputteringprocess after a cleaning process in order to remove organic materialsand alien substances from the substrate 1, thereby enhancing adhesionbetween the substrate 1 and the metal layer. FIG. 2A shows a step forforming a gate electrode 26 and a first capacitor electrode 22 bypatterning the first metal layer using a first mask.

A low resistance metal such as aluminum is used to form the gateelectrode 26 so as to reduce the RC delay. However, pure aluminum hasweak resistance to most enchants and may result in line defects due to aformation of a hillock during a high temperature process. Thus, analuminum alloy is used. And in some cases, a double layered gate is usedwherein another metal layer covers the aluminum or aluminum alloy.

A gate insulating layer 50 is deposited on the whole surface of thesubstrate 1 covering the gate and capacitor electrodes 26 and 22. Then,a pure amorphous silicon (a-Si:H) layer 52 and a doped amorphous silicon(n⁺a-Si:H) layer 54 are deposited sequentially on the gate insulatinglayer 50.

As shown in FIG. 2B, an active layer 55 and a semiconductor island 53are formed by patterning the silicon layers 52 and 54 using a secondmask. The doped amorphous silicon layer 54 (i.e. ohmic contact layer)reduces the contact resistance between the active layer 55 and anelectrode which is formed later.

FIG. 2C shows a step for forming a data line 24, source and drainelectrodes 28 and 30 by depositing a second metal layer. At the sametime, a second capacitor electrode 58 is formed on the gate insulatinglayer 50, covering a portion of the first capacitor electrode 22.

Then, the ohmic contact layer between the source and drain electrodes 28and 30 is etched using the source and drain electrodes 28 and 30 as amask.

As depicted in FIG. 2D, an insulating layer is deposited on the entiresurface of the substrate 1 covering the source and drain electrodes 28and 30. The insulating layer is patterned using a fourth mask to form aprotection film 56. The protection film 56 may be selected frominorganic materials such as SiN_(x) and SiO₂ or organic materials suchas a BCB (benzocyclobutene). In addition, a material having a high lighttransmittance, humidity resistance and durability is used to form theprotection film 56 in order to protect the channel area of the TFT andmajor portions of a pixel region from possible exposure to humidity andscratches during later processing steps.

Further, a data pad contact hole 33 is formed on the data pad 23, anddrain and capacitor contact holes 31 and 59 are formed on the drainelectrode 30 and the second capacitor electrode 58, respectively.

FIG. 2E shows a step for forming a pixel electrode 14 by depositing atransparent conducting oxide (TCO) layer 15 and patterning it using afifth mask. Indium tin oxide (ITO) is usually employed for thetransparent conducting oxide layer. The pixel electrode 14 contacts thesecond capacitor electrode 58 through the capacitor contact hole 59 andthe drain electrode 30 through the drain contact hole 31. Anotherportion of the transparent conducting oxide layer 15 is also formedcontacting the data pad 23 through the data pad contact hole 33.

As described, the conventional art requires at least five masks infabricating the TFT array panel of the LCD device, and each mask processrequires many steps such as cleaning, depositing, baking and etching.Therefore, if the number of mask processes is reduced, even if only byone, then production would be increased and cost would be decreased.

SUMMARY OF THE INVENTION

Therefore it is an object of the present invention to provide a thinfilm transistor array panel of a liquid crystal display device andmethods of forming the same that eliminates the problems of conventionalmethods.

A further object of the present invention is to fabricate the liquidcrystal display device with a high yield and a reduced fabrication time.The present invention provides, in one embodiment, a method forfabricating a liquid crystal display array panel, comprising the stepsof: forming a gate line by depositing a first metal layer on a substrateand patterning the first metal layer using a first mask; depositing aninsulating layer, a pure amorphous silicon layer, a doped amorphoussilicon layer and a second metal layer sequentially on the entiresurface of the substrate and covering the gate line; forming a data lineregion, a gate line protection layer and an active area by patterningthe second metal layer and the doped amorphous silicon layer using asecond mask, the data line region having a source electrode and the gateline protection layer having a drain electrode spaced at a predetermineddistance from the source electrode; depositing a protection layer on theentire surface of the substrate while covering the data line region, thegate line protection layer and the active area; forming a data line, aprotection film and a gate insulating layer using a third mask;depositing a transparent conductive material on the entire surface ofthe substrate while covering the data line and the source and drainelectrodes; and forming a pixel electrode and exposing a portion of thegate line using a fourth mask, the pixel electrode being connected withthe drain electrode, the exposed portion extending from the active area.

The present invention provides, in another embodiment, a method forfabricating a liquid crystal display device, comprising steps of:forming a gate line by depositing a first metal layer on a substrate andpatterning the first metal layer using a first mask; depositing aninsulating layer, a pure amorphous silicon layer, a doped amorphoussilicon layer and a second metal layer sequentially on the entiresurface of the substrate and covering the gate line; forming an activearea using a second mask by selectively patterning the second metallayer and the pure amorphous silicon layer, the second metal layercovering the entire surface of the substrate except for the active area;depositing a protection layer on the entire surface of the substratewhile covering the data line region, the gate line protection layer andthe active area; forming a data line, a protection film, a gateinsulating layer, and source and drain electrodes by patterning thesecond metal layer, the pure amorphous metal layer, the doped amorphoussilicon layer and the insulating layer using a third mask; depositing atransparent conductive material on the entire surface while covering thedata line and the source and drain electrodes; and forming a pixelelectrode and exposing a portion of the gate line using a fourth mask,the pixel electrode being connected with the drain electrode, theexposed portion extending from the active area.

The present invention provides, in a further embodiment, a method forfabricating a liquid crystal display device, comprising steps of:forming a gate line by depositing a first metal layer on a substrate andpatterning the first metal layer using a first mask; depositing aninsulating layer, a pure amorphous silicon layer, a doped amorphoussilicon layer and a second metal layer sequentially on the entiresurface of the substrate while covering the gate line; forming an activearea and a data line region using a second mask by selectivelypatterning the second metal layer and the pure amorphous silicon layer,the second metal layer away from the data line region and covering theentire surface of the substrate excluding the active area and the dataline; depositing a protection layer on the entire surface of thesubstrate and covering the data line region, the gate line protectionlayer and the active area; forming a data line, a protection film, agate insulating layer, and source and drain electrodes using a thirdmask by patterning the second metal layer, the pure amorphous metallayer, the doped amorphous silicon layer and the insulating layer;depositing a transparent conductive material on the entire surfaceincluding the data line and the source and drain electrodes; and forminga pixel electrode and exposing a portion of the gate line using a fourthmask, the pixel electrode being connected with the drain electrode, theexposed portion extending from the active area.

The first metal layer can be anyone of Cr, Mo, and an aluminum-basedmetal. The present invention provides a method further comprising, astep of removing the exposed portion of the gate line. The transparentconductive material is Indium Zinc Oxide. In the third mask process isformed a contact hole to connect the drain electrode with the pixelelectrode. A contacting area between the drain electrode and the pixelelectrode is larger than a cross section area of the drain electrode.The active area has a “C” shape.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the invention willbecome more readily apparent from a consideration of the followingdetailed description set forth with reference to the accompanyingdrawings, which specify and show preferred embodiments of the inventionof which:

FIG. 1 is a plan view of a conventional LCD device;

FIGS. 2A to 2E are sectional views taken along lines A—A and B—B of FIG.1, and illustrate fabrication process steps of an active matrix liquidcrystal display device according to the prior art;

FIG. 3 is a plan view illustrating a liquid crystal display (LCD) deviceof the invention;

FIGS. 4A to 4D are plan views illustrating fabrication process steps ofa liquid crystal display (LCD) device according to an embodiment of theinvention;

FIGS. 5A to 5D are sectional views illustrating fabrication processsteps of a liquid crystal display (LCD) device according to theembodiment of the invention;

FIG. 6 is a cross sectional view illustrating a connection state betweenthe drain electrode and the pixel electrode according to the embodimentof the invention;

FIG. 7A is a sectional view illustrating another structure of connectingthe drain electrode with the pixel electrode;

FIG. 7B is a plan view of FIG. 7A;

FIG. 8 is a plan view illustrating a modified second mask process stepaccording to the invention; and

FIG. 9 is a plan view illustrating another modified second mask processstep according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, an example of which is illustrated in theaccompanying drawings.

FIG. 3 is a plan view illustrating a liquid crystal display (LCD) devicefabricated by a method according to an embodiment of the invention. Thefabrication steps will be explained with reference to FIGS. 4A to 4D andFIGS. 5A to 5D, which are plan views and sectional views taken alongline V—V of FIG. 3, respectively.

First, FIGS. 4A and 5A show a process step to form a gate line on asubstrate 1 using a first mask.

A gate line 100 having a gate electrode 102 is formed by depositing andpatterning a first metal layer. A metal such as Cr and Mo may be used asthe first metal layer, but an aluminum-based alloy metal with duallayered structure of AlNd and Mo is preferred. Though the gate electrodeis defined as a portion 102 in the gate line 100, the gate electrode 102can be formed to protrude from the gate line 100.

As shown in FIGS. 4B and 5B, an insulating layer 151, a pure amorphoussilicon layer 152, a doped amorphous silicon layer 154 and a secondmetal layer 156 are deposited sequentially on the entire surface of thesubstrate 1 including the gate line 100. The second metal layer 156 andthe doped amorphous silicon layer 154 are patterned using a second maskto form a data line region 104, a gate line protection layer 106, anddrain and source electrodes 108 and 110. In this step, an active area orchannel 101 of a thin film transistor (TFT) is defined between the drainelectrode 108 and the source electrode 110. The gate line protectionlayer 106 is larger in width than the gate electrode 100, and is used toprotect a predetermined portion of the insulating layer 151 whichprotects the gate electrode 100 from being damaged in a later etchingprocess.

As shown in FIGS. 4C and 5C, a protection layer 158 is deposited on theentire substrate 1 and covers the data line region 104 and the gate lineprotection layer 106. Then, a third mask process is performed to formprotection films 160 and 162, a data line 112 and a gate insulatinglayer 150, which covers the gate electrode 100. At this point, theetched region after the third mask process can be defined as two regions“A” and “C”. The region “A” is etched so that the substrate 1 isexposed, and the region “C” is etched so that the gate insulating layer150 is exposed. Layers on the two regions “A” and “C” are etchedsimultaneously. As explained above, the gate insulating layer 150prevents the gate line 100 from being damaged.

As shown in FIGS. 4D and 5D, a pixel electrode 116 is formed using afourth mask. A transparent conductive material is employed for the pixelelectrode 116. Indium Zinc Oxide (IZO) is preferred due to its goodlight transmittance characteristics.

To form a storage capacitor “S”, the pixel electrode 116 is formed tooverlap a portion of the gate line 100. Namely, the gate line 100 servesas a first capacitor electrode, the pixel electrode 116 serves as asecond capacitor electrode, and the gate insulating layer 150 betweenthe gate line 100 and the pixel electrode 116 functions as a dielectriclayer. Therefore, an overlapping portion of the pixel electrode 116 andthe gate line 100 constitute the storage capacitor “S”.

Further, a portion 120 of the gate line 100 extending from the activearea 101 should be exposed when the pixel electrode 116 is formed withthe fourth mask. This prevents a short between the gate electrode 100and the active area 101 from occurring. At this time, during the fourthmask process, the exposed portion 120 is affected by a developer ordeveloping solution. Thus, if the gate line 100 is made ofaluminum-based metal, which has weak resistance to the developer, theexposed portion 120 is preferably etched by a developer after the fourthmask process in order to prevent a short between the exposed portion 120and the active area 101. But the exposed portion 120 need not be removedif the gate line 100 is made of a chromium-based metal, which has a highcorrosion resistance.

FIG. 6 is a sectional view illustrating a connection between the drainelectrode 108 and the pixel electrode 116 along the line VI—VI in FIG.4D. As shown in FIG. 6, the drain electrode 108 connects with the pixelelectrode 116 at a portion “Z” through a contact hole 114, which isformed during the third mask process shown in FIG. 4C. Thus, a length ofa contacting surface between the drain electrode 108 and the pixelelectrode 116 is determined by the dimensions of the contact hole 114.

As shown in FIGS. 7A and 7B, the drain electrode 108 has a sloped endportion 108 a, which may be concave or convex and the end portion 108 adirectly contacts the pixel electrode 116. In this case, the contactingsurface between the drain and pixel electrodes 108 and 116 is largerthan a width of the drain electrode 108, which in turn lowers contactresistance.

FIG. 8 shows another method for the second mask process. In FIGS. 4B and5B, after the second metal layer 156 is deposited on the doped siliconlayer 154, portions of the metal layer 156 and the doped silicon layer154 are etched using the second mask except for the data line region 104and the gate line protection layer 106. On the other hand, as shown inFIG. 8, during the second mask process only a channel region or activearea 101 of the metal layer 156 is etched. In this case, only the activearea is formed using a second mask, and the data region 104 and the gateline protection layer 106 are not formed.

FIG. 9 shows yet another method for the second mask process of theinvention. As shown in FIG. 9, the second metal layer 156 is depositedand patterned to form the data line region 104 and the gate lineprotection layer 106 spaced apart from the data line region 104. Thechannel area 101 is also formed during this second mask process.

The third and fourth mask processes shown in FIGS. 5C and 5D are alsoadapted to fabricate the LCD device for the modified processes as shownin FIGS. 8 and 9.

The embodiments of the invention have the following advantages. Themanufacture of the liquid crystal display device is accomplished usingfewer mask steps, thus, the fabrication time and the cost are reduced,which leads to high yield with less misalignment.

Since the data line is formed at the same time when the protection layeris patterned, a width of the data line can be controlled.

Further, since the pixel electrode and the gateline act as electrodes ofa capacitor, a separate intervening conductive layer (as in theconventional art) is not needed, i.e., can be eliminated.

It is understood that various other modifications will be apparent toand can be readily made by those skilled in the art without departingfrom the scope and spirit of this invention. Accordingly, it is notintended that the scope of the claims appended hereto be limited to thedescription as set forth herein, but rather that the claims be construedas encompassing all the features of patentable novelty that reside inthe present invention, including all features that would be treated asequivalents thereof by those skilled in the art to which this inventionpertains.

What is claimed is:
 1. A method for fabricating a liquid crystal displayarray panel, comprising: forming a gate line by depositing a first metallayer on a substrate and patterning the first metal layer using a firstmask; depositing an insulating layer, a pure amorphous silicon layer, adoped amorphous silicon layer and a second metal layer sequentially onthe entire surface of the substrate and covering the gate line; forminga data line region, a gate line protection layer and an active area bypatterning the second metal layer and the doped amorphous silicon layerusing a second mask, the data line region having a source electrode andthe gate line protection layer having a drain electrode spaced at apredetermined distance from the source electrode; depositing aprotection layer on the entire surface of the substrate while coveringthe data line region, the gate line protection layer and the activearea; forming a data line, a protection film and a gate insulating layerusing a third mask; depositing a transparent conductive material on theentire surface of the substrate while covering the data line and thesource and drain electrodes; and forming a pixel electrode and exposinga portion of the gate line between edges of the data line region and thedrain electrode using a fourth mask, the pixel electrode being connectedwith the drain electrode, the exposed portion extending from the activearea.
 2. The method according to claim 1, wherein the first metal layeris one of Cr, Mo, and aluminum-based metal.
 3. The method according toclaim 1, further comprising, a step of removing the exposed portion ofthe gate line.
 4. The method according to claim 1, wherein thetransparent conductive material is Indium Zinc Oxide.
 5. The methodaccording to claim 1, wherein in the third mask process a contact holeis formed to connect the drain electrode with the pixel electrode. 6.The method according to claim 1, wherein a contact area between thedrain electrode and the pixel electrode is larger than a cross sectionarea of the drain electrode.
 7. The method according to claim 1, whereinthe active area has a “C” shape.
 8. A method for fabricating a liquidcrystal display device, comprising steps of: forming a gate line bydepositing a first metal layer on a substrate and patterning the firstmetal layer using a first mask; depositing an insulating layer, a pureamorphous silicon layer, a doped amorphous silicon layer and a secondmetal layer sequentially on the entire surface of the substrate andcovering the gate line; forming an active area using a second mask byselectively patterning the second metal layer and the pure amorphoussilicon layer, the second metal layer covering the entire surface of thesubstrate except for the active area; depositing a protection layer onthe entire surface of the substrate and covering the data line region,the gate line protection layer and the active area; forming a data line,a protection film, a gate insulating layer, and source and drainelectrodes by patterning the second metal layer, the pure amorphousmetal layer, the doped amorphous silicon layer and the insulating layerusing a third mask; depositing a transparent conductive material on theentire surface while covering the data line and the source and drainelectrodes; and forming a pixel electrode and exposing a portion of thegate line between edges of the data line region and the drain electrodeusing a fourth mask, the pixel electrode being connected with the drainelectrode, the exposed portion extending from the active area.
 9. Themethod according to claim 8, wherein the first metal layer is one of Cr,Mo, and aluminum-based metal.
 10. The method according to claim 8,further comprising, a step of removing the exposed portion of the gateline.
 11. The method according to claim 8, wherein the transparentconductive material is Indium Zinc Oxide.
 12. The method according toclaim 8, wherein a contact area between the drain electrode and thepixel electrode is larger than a cross section area of the drainelectrode.
 13. The method according to claim 8, wherein in the thirdmask process a contact hole is formed to connect the drain electrodewith the pixel electrode.
 14. The method according to claim 8, whereinthe active area has a “C” shape.
 15. A method for fabricating a liquidcrystal display device, comprising steps of: forming a gate line bydepositing a first metal layer on a substrate and patterning the firstmetal layer using a first mask; depositing an insulating layer, a pureamorphous silicon layer, a doped amorphous silicon layer and a secondmetal layer sequentially on the entire surface of the substrate andcovering the gate line; forming an active area and a data line regionusing a second mask by selectively patterning the second metal layer andthe pure amorphous silicon layer, the second metal layer away from thedata line region and covering the entire surface of the substrateexcluding the active area and the data line; depositing a protectionlayer on the entire surface of the substrate while covering the dataline region, the gate line protection layer and the active area; forminga data line, a protection film, a gate insulating layer, and source anddrain electrodes using a third mask by patterning the second metallayer, the pure amorphous metal layer, the doped amorphous silicon layerand the insulating layer; depositing a transparent conductive materialon the entire surface including the data line and the source and drainelectrodes; and forming a pixel electrode and exposing a portion of thegate line between edges of the data line region and the drain electrodeusing a fourth mask, the pixel electrode being connected with the drainelectrode, the exposed portion extending from the active area.
 16. Themethod according to claim 15, wherein the first metal layer is one ofCr, Mo, and aluminum-based metal.
 17. The method according to claim 16,further comprising, a step of removing the exposed portion of the gateline.
 18. The method according to claim 15, wherein the transparentconductive material is Indium Zinc Oxide.
 19. The method according toclaim 15, wherein in the third mask process is formed a contact hole toconnect the drain electrode with the pixel electrode.
 20. The methodaccording to claim 15, wherein a contacting area between the drainelectrode and the pixel electrode is larger than a cross section area ofthe drain electrode.
 21. The method according to claim 15, wherein theactive area has a “C” shape.